1. Field of the Invention
The present invention relates to an optical information playback apparatus, and more particularly to an apparatus for playing back information from a so-called optical disk.
2. Description of the Prior Art
An apparatus has been proposed in which a guide groove is formed on an optical disk and information to be additionally recorded is recorded by tracking a light spot along the guide groove while modulating a light intensity of the light spot. One example thereof is an apparatus disclosed in Japanese Patent Application Laid-Open (Kokai) No. 130102/79, in which the guide groove is of phase type having a groove depth of .lambda./8 (where .lambda. is a wavelength of a playback laser beam). The additional information is recorded in intensity type in which pits are formed in a metal thin film vapor-deposited on the surface of the disk.
In such a prior art apparatus, auto-focusing means for focusing the laser beam onto the disk and tracking means for controlling the position of a converging point (spot) of the laser beam such that the laser beam tracks a desired groove on the disk are required. Apparatus which meet the above requirements are illustrated in FIGS. 1 and 2. In FIG. 1, a laser beam 2 emitted from a laser light source 1 passes through a beam splitter 3, a light deflector 20 and a converging lens 4 and then it is focused onto a track 5 of an optical disk 6 (only a portion of which is shown in FIG. 1). A reflected laser beam passes through the converging lens 4 and the light deflector 20 and then it is directed to a unidirectional lens 7 such as a cylindrical lens by the beam splitter 3. The cylindrical lens 7 is arranged with an angle of approximately 45 degrees with respect to the direction of the track, that is, y-direction in FIG. 1. Similarly, a light detector 11 (to which light transmitted through the cylindrical lens 7 is directed) is arranged such that one of border lines l and l' (that is, orthogonal axes which divide a light receiving surface into four sections) on the light receiving surface is in parallel to the y-direction as shown in FIG. 2. With this arrangement, if the depth of the pit deviates from one quarter of a wavelength of the playback laser beam, the distribution of the reflected light at the track is asymmetric around the track and an effect thereof does not reflect to a focusing signal.
In FIG. 2, the focusing signal is produced as a result of a change in the distribution on the light receiving surface (divided into four sections) of the light detector 11 for detecting off-focusing as shown by a broken line in FIG. 2. That is, it is produced based on a difference signal (at a subtractor 14) between a sum signal (at an adder 12) of outputs from the light receiving surface sections 11a and 11b, and a sum signal (at an adder 13) of outputs from the light receiving surface sections 11c and 11d. The difference signal is applied to a voice coil 4 to effect autofocusing. On the other hand, the tracking signal is produced based on a difference signal (at a subtractor 18) between a sum signal (at an adder 28) of outputs of the light receiving surface sections 11a and 11d, and a sum signal (at an adder 29) of outputs from the light receiving surface sections 11b and 11c. The resulting difference signal represents the off-track signal due to the distribution of the defracted light. In FIG. 2, numerals 12, 13, 28 and 29 denote the adders, 14 and 18 denote the differential amplifiers, 15 denotes a phase compensator, 19 denotes a light deflection drive circuit, and 20 denotes a tracking deflector.
The method of deriving the tracking signal in this manner has a disadvantage of a low S/N ratio.